Parkinson's disease is the second most common neurodegenerative disorder, affecting nearly 1 million people in North America. The disease is characterized by symptoms such as muscle rigidity, tremor and bradykinesia.
Early studies of Parkinson's disease showed unusual inclusions in the cytoplasm of neurons (i.e., Lewy bodies), occurring predominantly in the substantia nigra, which innervate the striatal region of the forebrain. Although Lewy bodies were also found in other neurodegenerative conditions, the presence of Lewy bodies in Parkinson's disease is accompanied by cell loss in the substantia nigra. This cell loss is considered to be the defining pathological feature of Parkinson's disease.
Epidemiological studies have reported geographic variation in Parkinson's disease incidence, leading to the search for environmental factors (Olanow and Tatton, Ann. Rev. Neurosci., 22:123-144 [1998]). The recent discovery that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxin causes a Parkinson's-like syndrome indistinguishable from the idiopathic disease suggests that Parkinson's disease may be caused by environmental factors (e.g., toxins and causative agents). (See e.g., Langston, Ann. Neurol., 44:S45-S52 [1998]).
Recent research has also identified genes associated with Parkinson's disease (Mizuno et al., Biomed. Pharmacother., 53(3):109-116 [1999]; Dunnett and Bjorklund, Nature 399 (6738 Suppl):A32-A39 [1999]); namely, the .alpha.-synuclein gene (Polymeropouos et al., Science 276:2045-2047 [1997]), the parkin gene (Kitada et al., Nature 392:605-608 [1998]), and the UCH-L1 thiol protease gene (Leroy et al., Nature 395:451-452 [1998]). Although additional chromosomal loci associated with the disease state have been identified, these chromosomal loci have not been analyzed at the molecular level. At present, the biochemical roles played by these gene products in both normal cells and in diseased neurons remain ambiguous, and no gene therapy protocols involving their use have been developed.
Furthermore, Parkinson's disease is associated with the progressive loss of dopamine neurons in the ventral mesencephalon of the substantia nigra (Shoulson, Science 282: 1072-1074 [1998]), which innervates the major motor-control center of the forebrain, the striatum. Although a gradual decline in the number of neurons and dopamine content of the basal ganglia is normally associated with increasing age, progressive dopamine loss is pronounced in people suffering from Parkinson's disease, resulting in the appearance of symptoms when about 70-80% of striatal dopamine and 50% of nigral dopamine neurons are lost (Dunnett and Bjorklund, supra). This loss of dopamine-producing neurons resulting in a dopamine deficiency is believed to be responsible for the motor symptoms of Parkinson's disease.
Although the cause of doparninergic cell death remains unknown, it is believed that dopaminergic cell death is affected by a combination of necrotic and apoptotic cell death. Mechanisms and signals responsible for the progressive degeneration of nigral dopamine neurons in Parkinson's disease have been proposed (Olanow et al., Ann. Neurol., 44:S1-S196 [1998]), and include oxidative stress (from the generation of reactive oxygen species), mitochondrial dysfunction, excitotoxicity, calcium imbalance, inflammatory changes and apoptosis as contributory and interdependent factors in Parkinson's disease neuronal cell death.
Apoptosis (i.e., programmed cell death) plays a fundamental role in the development of the nervous system (Oppenheim, Ann. Rev. Neurosci., 14: 453-501 [1991]), and accelerated apoptosis is believed to underlie many neurodegenerative diseases, including Parkinson's disease (Barinaga, Science 281: 1303-1304 [1998]; Mochizuki et al., J. Neurol. Sci., 137: 120-123 [1996]; and Oo et al., Neuroscience 69: 893-901 [1995]). In living systems, apoptotic death can be initiated by a variety of external stimuli, and the biochemical nature of the intracellular apoptosis effectors is at least partially understood.
In light of the selective death of dopamine producing neurons, administration of L-dihydroxyphenylalanine (L-DOPA) remains the most widely used treatment of Parkinson's disease. However, the administration of therapeutically effective doses of L-DOPA is accompanied by disabling side effects. Furthermore, in some cases, treatment with L-DOPA requires the coadministration of a peripheral DOPA-decarboxylase inhibitor (e.g., carbidopa), which is also accompanied by adverse side effects.
Newer drug refinements and developments include direct-acting dopamine agonists, slow-release L-DOPA formulations, inhibitors of the dopamine degrading enzymes catechol-O-methyltransferase (COMT) and monoamine oxidase B (MAO-B), and dopamine transport blockers. These treatments enhance central dopaminergic neurotransmission during the early stages of Parkinson's disease, ameliorate symptoms associated with Parkinson's disease, and temporarily improve the quality of life. However, despite improvements in the use of L-DOPA for treating Parkinson's disease, the benefits accorded by these dopaminergic therapies are temporary, and their efficacy declines with disease progression. In addition, these treatments are accompanied by severe adverse motor and mental effects, most notably dyskinesias at peak dose and "on-off" fluctuations in drug effectiveness (Poewe and Granata, in Movement Disorders. Neurological Principles and Practice (Watts and Koller [eds]) McGraw-Hill, New York [1997]; and Marsden and Parkes, Lancet 1:345-349 [1977]). No drug treatments are currently available that lessen the progressive pace of nigrostriatal degeneration, postpone the onset of illness, or that substantively slow disability (Shoulson, supra).
Other methods for the treatment of Parkinson's disease involve neurosurgical intervention. The thalamic outputs of the basal ganglia are an effective lesion target for the control of tremor (i.e., thalamotomy). Despite the development of modem imaging and surgical techniques to improve the effectiveness of these neurosurgical interventions for the treatment of Parkinson's disease tremor symptoms, the use of neurosurgical therapies is not widely applicable. For example, thalamotomy does not alleviate the akinetic symptoms which are the major functional disability for many people suffering from Parkinson's disease (Marsden et al., Adv. Neurol., 74:143-147 [1997]).
Therapeutic methods aimed at controlling suspected causative factors associated with Parkinson's disease (e.g., therapies which control oxidative stress and excitotoxicity) have also been developed. Clinical trials have shown that administration of antioxidative agents vitamin E and deprenyl provided little or no neuroprotective function (Shoulson et al., Ann. Neurol., 43:318-325 [1998]). Glutamate-receptor blockers and neuronal nitric oxide synthase (NOS) inhibitors have been proposed as therapies for Parkinson's disease, however, no experimental results from human studies have yet been published (Rodriguez, Ann. Neurol., 44:S175-S188 [1998]).
The use of neurotrophic factors to stimulate neuronal repair, survival and growth in Parkinson's disease has also been studied, particularly the use of glial cell line-derived neurotrophic factor (GDNF). Although GDNF protein protects some dopamine neurons from death, it is difficult to supply GDNF protein to the brain. Furthermore, the use of such protein therapies in general is problematic, since protein molecules show rapid in vivo degradation, are unable to penetrate the blood-brain barrier and must be directly injected into the ventricles of the patient's brain (Palfi et al., Soc. Neurosci. Abstr., 24:41 [1998]; Hagg, Exp. Neurol., 149:183-192 [1998]; and Dunnett and Bjorklund, supra). Other neurotrophic factors which may have therapeutic value have been proposed based on in vitro and animal model systems, including neurturin, basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF), neurotrophins 3 and 4/5, ciliary neurotrophic factor and transforming growth factor .beta. (TGF-.beta.). However, the effectiveness of these therapies in humans remains unknown. At present, no single chemical compound or peptide has been reported to completely protect dopamine neurons from death by tropic factor withdrawal or neurotoxin exposure.
Cell replacement therapies have also received much attention as potential methods for treating Parkinson's disease (Freed et al., Arch. Neurol., 47:505-512 [1990]; Freed et al., N. Engl. J. Med., 327:1549-1555 [1992]; Lindvall et al., Science 247:574-577 [1990]; Spencer et al., N. Engl. J. Med., 327:1541-1548 [1992]; Widner et al., N. Engl. J. Med., 327:1556-1563 [1992]; Lindvall, NeuroReport 8:iii-x [1997]; Olanow et al., Adv. Neurol., 74:249-269 [1997]; and Lindvall, Nature Biotechn., 17:635-636 [1999]). These neural grafting therapies use dopamine supplied from cells implanted into the striatum as a substitute for nigrostriatal dopaminergic neurons that have been lost due to neurodegeneration. Although animal models and preliminary human clinical studies have shown that cell replacement therapies may be useful in the treatment of Parkinson's disease, the failure of the transplanted neurons to survive in the striatum is a major impediment in the development of cell replacement therapies.
Various sources of dopaminergic neurons for use in the transplantation process have been tried in animal experiments, including the use of mesencephalic dopamine neurons obtained from human embryo cadavers, immature neuronal precursor cells (i.e., neuronal stem cells), dopamine secreting non-neuronal cells, terminally differentiated teratocarcinoma-derived neuronal cell lines (Dunnett and Bjorkland, supra), genetically modified cells (Raymon et al., Exp. Neurol., 144:82-91 [1997]; and Kang, Mov. Dis., 13:59-72 [1998]), cells from cloned embryos (Zawada et al., Nature Medicine 4:569-573 [1998]) and xenogenic cells (Bjorklund et al., Nature 298:652-654 [1982]; Huffaker et al., Exp. Brain Res., 77:329-336 [1989]; Galpem et al., Exp. Neurol., 140:1-13 [1996]; Deacon et al., Nature Med., 3:350-353 [1997]; and Zawada et al., Nature Med., 4:569-573 [1998]). Nonetheless, in current grafting protocols, no more than 5-20% of the transplanted dopamine neurons survive.
As indicated above, currently used therapies are primarily directed at symptomatic relief, are often associated with debilitating side-effects, lose efficacy over time, are difficult to administer to the brain, and provide poor long term management of the disease. In addition, currently used cell replacement therapies involving grafting protocols have not been widely used due to the inability of transplanted cells to survive in the recipient. Thus, new methods for the treatment of Parkinson's disease that are effective and convenient, but lacking in significant side effects are needed. Furthermore, there is a need for methods that improve the viability of endogenous neurons in people suffering from Parkinson's disease. There is also a need for methods that improve the viability of transplanted neurons in patients who have undergone or are undergoing transplantation therapy.
The same considerations are involved in the treatment and management of neurodegenerative diseases other than Parkinson's disease. For example, Parkinson's disease shares many physiological and pathological characteristics with other neurodegenerative disorders, including Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease). In general, these neurodegenerative disorders are progressive (i.e., their symptoms are not apparent until months or more commonly years after the disease has begun), and caused by an initial reduction of neuronal function, followed by a complete loss of function upon neuronal death. In addition, these neurodegenerative disorders are characterized by the presence of protein aggregates that are believed to hamper cellular functions (e.g., neurotransmission), and may ultimately result in cell death (Sasaki et al., Am. J. Pathol., 153:1149-1155 [1998]). Indeed, apoptotic cell death seems to play a significant role in the neurodegenerative process.
Alzheimer's disease is the most common neurodegenerative disorder. Recent experimental evidence suggests that neuronal death in Alzheimer's may occur through apoptosis (Smale et al., Exp. Neurol. 133(2):225-230 [1995]; and Kim et al., Science 277:373-376 [1997]). Furthermore, in the postmortem brains of Alzheimer's patients, expression of the apoptosis-related transcriptional factor c-jun was colocalized within the cells that also contained DNA strand breaks characteristic of apoptosis (Anderson et al., J. Neurosci., 16:1710-1719 [1996]).
Huntington's disease is an autosomal dominant progressive neurodegenerative disorder resulting from a CAG/polyglutamine repeat expansion in the gene encoding this disease, ultimately resulting in the death of striatal neurons. The polyglutamine expansion results in the formation of insoluble, high molecular weight protein aggregates similar to those seen in Alzheimer's disease (Scherzinger et al., Cell 90:549-558 [1997]). Postmortem examination of the brains of patients suffering from Huntington's disease revealed that CAG repeat length positively correlates with the degree of DNA fragmentation within the afflicted striatum (Butterworth et al., Neurosci., 87:49-53 [1998]), indicating that neuronal degeneration observed in Huntington's disease may also occur through an apoptotic process.
Amyotrophic lateral sclerosis (ALS) is caused by a progressive degeneration of spinal cord motor neurons and results in complete paralysis, respiratory depression and death. Aggregates of ubiquitinated proteins have been observed in ALS (Kato et al., Histol. Histopathol., 14:973-989 [1999]). Recent experiments suggest that death of motor neurons in ALS may have an apoptotic component (Pasinelli et al., Proc. Natl. Acad. Sci. USA 95:15763-15768 [1998]; and Martin, J. Neuropathol. Exp. Neurol., 58:459-471 [1999]).
Currently used therapies for Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis suffer the same limitations associated with Parkinson's disease therapies described above (See e.g., Sramek et al., Drugs & Aging 14:359-373 [1999]; Mayeux and Sano, N. Eng. J. Med., 341:1670-1679 [1999]; Eisen and Weber, Drugs & Aging, 14:173-196 [1999]; Borasio et al., Neurology 51:583-586 [1998]; Riviere et al., Arch. Neurol., 55:526-528 [1998]; Rosas et al., Movement Dis., 14:326-330 [1999]; Kopyov et al., Exp. Neurol., 149:97-108 [1998]; and Haque et al., Mol. Med. Today 3:175-183 [1997]). These treatments are primarily directed at symptomatic relief, are often associated with severe side-effects, lose efficacy over time, are difficult to administer to the central nervous system, and provide poor long term disease management. Thus, new methods for the treatment of neurodegenerative diseases, including but not limited to Parkinson's disease, Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis, that are effective and convenient, but lacking in significant side effects are needed.